1. Field of the Invention
The invention relates to a control of an automatic transmission for a vehicle, and more particularly, to a control apparatus and method for an automatic transmission which executes a neutral control.
2. Description of the Related Art
An automatic transmission to be mounted in a vehicle includes a shifting mechanism which is connected to an engine via a torque converter and the like, and which has a plurality of power transmission paths. This automatic transmission automatically switches gear ratios (i.e., speeds) based on, for example, a throttle opening and vehicle speed. A vehicle having an automatic transmission is typically provided with a shift lever which is operated by a driver into any one of various shift positions (e.g., REVERSE, NEUTRAL, DRIVE). In automatic shift mode there is a “forward drive range” in which the automatic transmission is automatically switched to a predetermined gear ratio/speed.
When a vehicle having this type of automatic transmission is stopped with the shift lever in a position corresponding to a forward speed range, such as DRIVE, driving force from the idling engine is transmitted to the transmission via the torque converter and this force is then transmitted to the wheels, resulting in a phenomenon known as “creeping.” Creeping is extremely useful under certain conditions. For example, it helps to keep the vehicle from slipping backwards and enables a vehicle stopped on an incline to start smoothly. When the driver wants a vehicle that is stationary on a flat road to remain in one spot, however, creeping is unnecessary and must be suppressed by operating the brake. That is, the brake is used to suppress the creeping force from the engine and the fuel efficiency of the engine decreases a corresponding amount.
To improve fuel efficiency, therefore, it has been proposed to put a transmission that is in DRIVE or another forward speed range into a neutral state including a semi-neutral state that resembles neutral when the vehicle is being held stationary by the brake pedal being depressed so as to operate the brake and the accelerator being almost completely closed.
JP(A) 2001-349424 discloses a control apparatus which prohibits a transmission from moving into a neutral state while on an incline.
This control apparatus controls an automatic transmission with automatic neutral control, by which the transmission is put into a neutral state, when the automatic transmission is in DRIVE or another forward speed range and the vehicle has been stationary in the idling state for a predetermined sustained period of time. This control apparatus includes an automatic neutral state alert circuit which alerts the driver that the automatic transmission is in the neutral state, an incline detection circuit which detects when the vehicle is on an incline, and an automatic neutral control prohibiting circuit which prohibits automatic neutral control when it has been detected that the vehicle is on an incline.
With this control apparatus, the automatic neutral state alert circuit alerts the driver that automatic neutral control is being performed on the automatic transmission so the driver does not feel a sense of discomfort. Further, by prohibiting the automatic neutral control when the vehicle is on an incline, backward slipping of the vehicle is able to be suppressed.
The control apparatus disclosed in the above-mentioned publication, however, uses an incline sensor to sense whether the vehicle is on an incline. When this type of sensor is mounted in a vehicle, it is affected by vibrations from the engine and drive-line when the vehicle is stationary as well as when the vehicle is running. Because of this, the raw data picked up by the sensor can not be used as it is. Therefore, the raw data picked up by the sensor is typically processed and the angle of inclination is detected according to the processed data. This data processing will hereinafter be described in a case in which a G sensor (i.e., an acceleration sensor), for example, is used as the incline sensor.
FIGS. 6 and 7 are graphs showing raw data (indicated by broken lines) picked up by the G sensor, and the processed data (indicated by solid lines). FIG. 6 shows a case in which the vehicle is on a flat road and FIG. 7 shows a case in which the vehicle is on an incline. The horizontal axes in both drawings represent time and the vertical axes represent acceleration. In the graphs, the vehicle comes to a stop with the passage of time. The output values of the raw data (i.e., the broken lines) shown in both FIGS. 6 and 7 are not exactly accurate as they are affected by vibrations and the like while the vehicle is driving, while the vehicle is coming to a stop, and while the vehicle is stopped. In particular, the vehicle vibrates a great deal just before and just after stopping, and a small value indicative of a flat road is output just after the vehicle is stopped on an incline.
Therefore, the raw data is typically put through a smoothing process, and the processed data (the solid lines) is used as a condition for starting the neutral control. In this smoothing process, at each sampling time, the effects from the raw data picked up by the G sensor on the processed data for the last sampling time are taken into consideration (Only about 30 to 80%, instead of 100%, of the raw data is taken into consideration at this time), and new processed data is created.
As a result, even when there is vibration in the raw data, the effects are reduced, enabling the error to be kept small. According to this smoothing process, values of the processed data that correspond to the true gradient of the road are output approximately one second after the vehicle has come to a stop. This is evident from the fact that the processed data (the solid lines) is temporally behind the raw data (the broken lines) in FIGS. 6 and 7.
In JP(A) 2001-349424, this kind of smoothing process is not mentioned, but when a G sensor, which is actually the most common incline sensor, is used to obtain the inclination of the road on which the vehicle is stopped, or further, when any type of sensor is used to obtain the inclination of the road on which the vehicle is stopped, this type of time delay is unavoidable. That is, in reality, even if the conditions for starting neutral control are fulfilled in a condition other than the condition that the road on which the vehicle is stopped is not an incline, it is not possible to determine whether the vehicle is stopped on an incline for approximately one second due to a delay in the output of the processed data from the G sensor.
Therefore, even when the vehicle is stopped on a flat road, in which case the neutral control can start immediately (i.e., even if the data is in a region below the level of the incline determination threshold value shown in FIG. 6), the neutral control starts after confirming that the processed data output from the G sensor approximately one second after the vehicle has come to a stop is below the level of the incline determination threshold. Accordingly, there is a delay in the start of the neutral control. On the other hand, when the vehicle is stopped on an incline, in which case the neutral control should not be started (i.e., even if the data is in a region above the level of the incline determination threshold value shown in FIG. 7), it is determined that the neutral control can be started because the processed data output from the G sensor immediately after the vehicle was stopped is below the level of the incline determination threshold value. Because the vehicle is stopped on an incline, however, the neutral control should not be started. Therefore, it is necessary to start the neutral control after confirming that the processed data output from the G sensor after approximately one second after the vehicle has stopped is truly below the level of the incline determination threshold value.
In this case, one way to eliminate the delay in the start timing of the neutral control caused by the time delay from the G sensor and substantially increase the time that the transmission is in the neutral state would be to increase the release speed of the input clutch of the automatic transmission, the release of which results in the transmission becoming in the neutral state. Doing so, however, would result in a large shock when the input clutch is released. With neutral control which is not based on an operation by the driver, the shift into the neutral state should be unnoticeable by the driver. Therefore, the input clutch used to achieve the neutral state should be released gradually, not immediately after a command to release it is output. The time it takes for the input clutch to completely release after this input clutch release command is output is controlled by an ECU (Electronic Control Unit).